The present invention relates generally to the field of fiber optic communications and specifically to the separation of wavelengths by dispersion using wavelength multiplexers.
The use of wavelength multiplexers (MUX) to combine (multiplex) and separate (demultiplex) wavelengths within light signals is well known in the field of fiber optic communications, especially in wavelength division multiplexing (WDM) systems. Wavelength division multiplexing refers to the transmission of two or more light signals over a common path using a different wavelength for each signal.
FIG. 1 depicts a conventional, free-space wavelength MUX device. The figure shows the device performing a demultiplexing operation. An input waveguide 20 introduces a light signal 10 carrying multiple signals of different wavelengths. Light signal 10 is collimated by a first pass through a lens 30. The light then strikes a planar diffraction grating 40 at the back focal plane which disperses the separate wavelength signals and reflects them. Each wavelength signal leaves the grating 40 at a distinct angle. The separated light signals 50 are then focused by a second pass through the lens 30 onto an output plane 60 which is perpendicular to the direction of propagation such that each wavelength signal creates an optical focal spot whose position is linearly related to the optical wavelength. A linear array 70 of one or more output waveguides 80 is positioned at output plane 60 such that each wavelength signal illuminates, and is optically coupled to, only one output waveguide 80.
A major limitation of this conventional multiplexer arrangement is its low tolerance for displacement of the input and output waveguides in a direction orthogonal to both the direction of dispersion and to the optical axis. FIG. 2a depicts the conventional multiplexer arrangement of FIG. 1 from an overhead view. FIG. 2b depicts the conventional multiplexer arrangement of FIG. 1, without the planar diffraction grating 40, from a frontal view. FIG. 2c depicts the same view as FIG. 2b, with the position of the input waveguide displaced in a direction orthogonal to both the direction of dispersion and to the optical axis and the position of the output waveguides displaced to match the input.
Referring to FIG. 2c, if the input waveguide is displaced to a position Ai, the output waveguides must be displaced in the opposite direction to a position Ao so as to allow them to continue to receive the separated light signals which originated from this input. The same is true if the input waveguide is moved in the other orthogonal direction to position Bi. The output waveguides must again be moved in the direction opposite to a position Bo so as to allow them to continue to receive the separated light signals which originated from the input. Thus, where the input waveguide is displaced in an orthogonal direction as shown in FIG. 2c, for example as caused by mechanical or thermal effects, the output waveguides must be realigned with the input otherwise system performance will suffer.
The present invention consists of a new method and apparatus for multiplexing and demultiplexing light signals of multiple wavelengths using two canceling dispersion stages with a stage between them which transforms the angular positions of the beams or the lateral positions of the beams or both the angular and lateral positions of the beams.
In the present invention, a light signal of multiple wavelengths enters through an input waveguide. The signal is then dispersed by a first stage of the invention. The dispersed signals of wavelengths then enter a second stage of the invention which transforms the wavelengths. The transformed signals of wavelengths then enter a third stage of the invention which performs the inverse dispersion of the first stage. In other words, if this third stage were placed immediately after the first stage, the third stage would nullify the dispersion performed by the first stage. However, adding the intervening transformation step results in only the simple linear dispersion being canceled in the system while the transformed part of the dispersion is retained.
In specific examples of the invention, this cancellation of simple linear dispersion and retention of the transformed part of the dispersion results in a much greater tolerance of displacement of the input or output waveguides in a direction orthogonal to both the direction of dispersion and to the optical axis as compared to the conventional arrangement. In these examples of the invention, the added intervening transformation step causes the resulting wavelengths which are outputted from the third stage to remain coplanar with the input waveguide despite any displacement of the input waveguide in a direction orthogonal to both the direction of dispersion and the optical axis.